1. Field of the Invention
The present invention relates to a case for electronic units and more particularly to a case for electronic units which is especially effective when used as a case for such units as input units, output units and a CPU unit forming a programmable controller.
2. Description of the Prior Art
Referring to FIG. 9, there is illustrated a perspective view showing a conventional programmable controller (hereinafter referred to as a PC) by way of example. In the Figure, numeral 1 designates a box-type case made from a synthetic resin material and opened at its front face, 2 a power supply unit inserted in the case 1, and 3 a CPU Unit screwed to the case 1 to adjoin the power supply unit 2, with control program setting means 5 (hereinafter referred to as setting means) including a connector 6 adapted for connection to a connection portion 4 of the CPU unit 3 being mounted on the power supply unit 2 and the CPU unit 3 and fixed in place with screws. Numeral 7 Designates input units, and 8 output units. The front panels of these units are screwed to the case 1 in juxtaposition to the CPU 3, and each of the units 7 and 8 includes a terminal board 9 and a signal indicator 10 which are arranged on the front panel for connection to a controlled system and a printed wiring board 11 which is attached to the back of the front panel and formed with the necessary electric circuit connected to a mother board (not shown) arranged at the bottom of the case 1.
With the PC including the above-mentioned component units, the necessary control program is preliminarily selected by the setting means 5 and it is stored in the CPU 3. The input signals from the controlled systrem are applied to the input units 7 and processed by the CPU unit 3 so that in accordance with the processing results, control signals are generated from the output units 8 thereby controlling the controlled system in accordance with the selected program.
While the PC has an excellent control function as mentioned above and it is presently used in a wide range of applications in the industrial world, it has the following disadvantages.
(1) The power supply unit 2, the CPU unit 3, the setting means 5, the input units 7 and the output units 8, forming the PC, are all contained in the case 1 and the numbers of these units (particularly the input units 7 and the output units 8) are varied in dependence on the intended use, the controlled system, etc. Therefore, cases 1 of various sizes (lengths) corresponding to such varying number must be prepared thus not only making the PC uneconomical but also increasing its cost.
(2) Since the input units 7 and the output units 8 are practically the same in external appearance thus making it difficult to draw a distinction therebetween and there is the danger of these units being mounted erroneously or operated erroneously. As a result, the types of units are indicated by letters, symbols or the like on the front panels or the whole or part of each front panel is painted in a different color by printing or the like in correspondence to the type of the unit and the inscription of letters or the like or the coloring operation is troublesome thus causing an increase in the cost and a problem from the design point of view.
(3) Since each of the units is fastened to the case 1 with two or more screws, the dismounting and mounting operations are very troublesome not only during the assemblage but also during the maintenance, inspection, repair, etc., thus requiring considerable time.
(4) The case 1 is generally made by molding a synthetic resin and it expands and contracts in response to temperature changes. On the other hand, attached to the bottom of the case 1 is the mother board made from a glass epoxy resin or the like which does not practially expand and contract in response to temperature changes and the printed wiring board of each unit is connected to the mother board through the connector. As a result, the case is expanded and contracted with changes in the ambient temperature thereby causing troubles between the mother board and the printed wiring boards.
(5) The terminal boards 9 and the printed wiring boards 11 provided on the input and output units 7 and 8 are exposed and thus dust and dirt tend to deposit on them thereby giving rise to the danger of causing any contact failure or short-circuit.
(6) If any particular unit is removed for the purpose of maintenance, inspection, adjustment or the like, a space is left in this portion so that there is the danger of the dust and dirt entering the case 1 through this space and causing any contact failure, short-circut or the like.
(7) The connecting operation of the ground terminals of the input and output units is troublesome, particularly where such units are mounted with greater density, the number of ground terminals is increased thus not only the cost is increased but also a considerable time is required for their connecting operation.